Sure, there's no story mode but the online experience is unmatched by any PSP game to date!
Just so you know, I am not a huge fan of the MGS series.
I have got MGS 2 but it is played very rarely.
I could not get my hands on Metal Gear Solid Portable Ops and so I picked up a copy of MPO+ instead.
I was rather disappointed that there is no story mode but the Infinity mission more or less makes up for this.
Basically you explore large maps in search for enemies. You have an enormous arsenal of weapons to unlock and use against them.
When you don't have weapons you can sneak up to someone and attack them in many various ways; Grab, Stagger, Throw down, Restrain, Hold-Up, Drag down, Restrain (a different kind), Drag, Choke, Shield, Interrogate and more!
The large choice of options you are given is fantastic.
You can kill someone or knock them out. If you choose to knock them out then while they are unconcious you can drag their bodies to your vehicle and by the end of your mission they will have joined your side and will be playable by you throughout the game until they get killed.
The graphics of this game are great (if you have MPO then you know what to expect) but textures close up can look very bland and messy.
NPCs and playable characters alike are very detailed and carefully constructed and so look very realistic!
The graphics are easily one of the best seen on a handheld gaming console.
Shadowing/lighting is great, eg when in first person mode, at some angles the Gun/firearm will glint/shine in the sunlight.
One brilliant addition to this game is Ragdoll Physics.
The music is just what you would expect from a stealth/action/shooter. They work well and fit in perfectly to what goes on in the gameplay.
Controls can feel quite quirky at first but eventually you should get the hang of them.
The camera view is brilliant, it follows your character always but you can adjust it with the directional button.
(Yes, you use the analog stick to move but while holding one of the trigger buttons, you use the directional buttons to walk).
You are put in first person mode once you hold the R button but the game goes under the third person category.
My favourite aspect of the game is online mode! It is large, exciting and packed with things to do!
There are many levels to unlock, surprising amounts of characters to unlock, many weapons/objects etc!
If you have MPO original, then you may want this to give you something more to do (you can load MPO data) and even if you haven't or have never even played a MGS game before, i would say give this a try because it's a whole new experience of gaming and it is great fun!
Remember, this expansion does not REQUIRE the first MPO game or even a gamesave!
I don't know if everyone will feel the same way about this game as me, but truth is, it is a great game!